For automotive integrated circuit devices, it is desirable that high test-coverage is achieved for both static and delay faults yet still keeping the test-cost under control to meet the stringent safety standards. As a measure to achieve close to zero defective parts per million for this class of devices, typically the test coverage targets are set as high as greater than 99% for static fault testing and greater than 95% delay fault testing. Static testing refers to detecting whether or not a given node in the design is fixed at the one or zero state (e.g., due to shorted node). Delay fault testing refers to measuring a transition event (e.g., response of a flip flop to a clock edge) to determine whether or not the transition event occurred in a desired timing window.
In standard scan designs, delay faults are typically tested using skewed-load also referred to as launch off shift (LOS) method or using broadside also referred to as a launch off capture method (LOC). Both methods launch transitions at the input of combinational block in different manner for the same fault detection. In the case of LOC, a scan-enable signal goes low post of last shift clock cycle and the launch and capture of the transition would happen along the functional path of the circuit. In LOS, the last shift clock is used to launch a transition from the scan-shift path onto the combinational block. In LOS, the fault activation path or scan path is controllable from the input of scan chain while in LOC, controllability of launching a transition at a fault site is less as it depends on the functional response of logic blocks to an initialize vector. Thus, LOS results in better fault coverage and less testing patterns than LOC. A disadvantage of LOS is that it requires the scan-enable signal to switch at-speed. Since last shift happens with a fast clock, the entire design under test will become active resulting in average power in the launch cycle to go very high. Also, the last shift at high speed clock will force additional timing requirements on an On chip Clock (OCC) controller in multi-clock domain design, for example. To solve these issues, a Launch on Extra/Extended Shift (LOES) method can be used, which launches a transition with the help of an extra fast shift clock. In LOES, extra shift and capture clocks are at-speed clocks. Essentially, loading unloading processes of LOES are similar to LOC whereas launching process is similar to LOS since the transition is launched through the shift path.